The present invention relates to a data converter and in particular to a data converter for use in a receiver or transmitter.
In many devices such as portable communication devices, especially those which use a digital modulation scheme, there is often a need to convert radio or other modulated signals received by the device from analogue to digital, and to convert signals to be transmitted by the device from digital to analogue.
Sigma-delta and delta-sigma converters are frequently used for this purpose for a number of well known reasons, such as good linearity, suitability for formation on an integrated circuit, etc. However, in order for such converters to be suitable for this purpose, especially for use in sophisticated portable communication devices, they generally need to be quite complex. Typically, such converters will include a number of different stages, with different levels of over-sampling at the different stages. Furthermore, complicated filtering is required at each stage to remove noise that has been generated by the converter. Typically this is achieved by providing a filter at the output of each stage which filters out at least all components whose frequency is greater than about half the clock frequency used at each stage.
Additionally, a device such as a portable communications device will need to perform digital signal processing on the signal after Analogue to Digital (A to D) conversion in the case of received signals, or prior to D to A conversion in the case of signals to be transmitted. Such signal processing will typically include some sort of phase function transformation, such as conversion between an Intermediate Frequency (IF) and base-band, and also digital filtering.
Furthermore, a conventional superheterodyne transceiver suffers from the drawback of only being able to receive or transmit within a single radio channel having a given channel bandwidth (e.g. 200 kHz in GSM), for a single setting of the main local oscillator. In order to change to a different channel, the main local oscillator needs to be reprogrammed to a new frequency. This requires a certain finite time, commonly referred to as the settling time, before the new frequency is correctly established. A settling time of 100-400 xcexcs is typical. This may be worse in some cases when, for example, it is necessary to adjust the dc offset of the transceiver when operating in the new channel.